In crown and bridge prosthodontics, metal copings are conventionally used to provide the essential structural strength and rigidity necessary for a dental restoration to resist the forces of mastication. In a ceramic-to-metal dental restoration, the metal coping forms the understructure, over which is applied a fired-on coating of porcelain or acrylic. A coating of porcelain is used over the coping for aesthetics and to simulate natural teeth. To the dental patient, color and the overall appearance of the dental restoration are critical factors in the satisfaction of the restoration. Accordingly, the color of the metal coping is important and should enhance the aesthetics of the restoration. For a ceramic-to-metal dental restoration, the metal coping should provide strength and background color.
In a co-pending application of Applicants entitled Moldable Dental Material and Method, Ser. No. 887,245, filed May 19, 1992, (now U.S. Pat. No. 5,234,343) a dental material composition is taught which can be readily shaped or molded into any desired shape for repairing and/or forming a dental restoration, without any investment or casting of the metal. The composition of the material and method of application is taught in Applicants' U.S. Pat. Nos. 4,742,861 and 4,990,394, the disclosure of which is herein incorporated by reference. In general, the dental material is composed of high- and low-fusing temperature metal particles combined in a matrix with a volatile binder for forming a dental restoration directly on a refractory die or model of the tooth or teeth to be restored. The material is shaped on the die into a desired configuration and heat-treated at a temperature to melt, or substantially melt, the low-fusing temperature metal particles and to volatize the binder, resulting in a porous, sponge-like structure having the shape it was given prior to heat treatment. A low-melting temperature filler material, preferably of gold, is then melted into the sponge-like structure to form a solid metal coping, with a configuration identical to the configuration of the shaped material on the refractory die before heat treatment and without experiencing distortion and/or shrinkage.
The solidified metal should possess a desirable color, which is reproducible with high accuracy, for use in forming a dental restoration. Heretofore, the process was sensitive to temperature and the atmosphere in the furnace. Even minor variations in the temperature during the heat treatment procedures would permit some oxidation of the metals to occur, which could deleteriously affect its color, and even more seriously, could inhibit the penetration and flow of filler material into the porous sponge, which would affect the size of the solidified structure. In fact, even the type of furnace used or its condition was able to affect the ability to accurately control the temperature during the heat treatment procedures. Although sophisticated furnace temperature control equipment is commercially available, the implementation of such equipment is costly and would be unacceptable to the dental practitioner. The sensitivity to temperature variation also limited the process to the fabrication within the furnace of one restoration at a time, unless even more elaborate temperature control measures were taken.